Second order nonlinear optical (NLO) polymers are expected to find extensive uses in opto-electronic applications. NLO polymers have several advantages over single crystalline inorganic and organic molecular systems. These include easy preparation, adjustable refractive indices and controlled arrangement of spatial order. For second order applications it is imperative that the material be noncentrosymmetric. In noncentrosymmetric organizations several organic molecular and polymeric systems have been characterized by large second order NLO coefficients, ultra-fast response times, performance over a broad wavelength range and high laser damage threshold compared to the traditional inorganic materials, e.g., lithium niobate (LiNbO.sub.3) or potassium dihydrogenphosphate (KH.sub.2 PO.sub.4). Background information relating to the principles of nonlinear optical polymers, is contained in "Nonlinear Optical and Electroactive Polymers", edited by Prasad and Ulrich, Plenum Press, (1988).
A number of applications, such as second harmonic generation (SHG), frequency mixing, electro-optic modulation, optical parametric emission, amplification and oscillation have been proposed for organic and polymeric materials with large second order NLO coefficients. R. D. Small et al., "Molecular and Polymeric Optoelectronic Materials: Fundamentals and Applications", edited by Khanarian, SPIE, 682:160 (1986). A number of approaches have been made in the past decade to organize NLO molecules in a polymer matrix in a noncentrosymmetric manner. The most important, but not the only aspect from the standpoint of application, is the organization of NLO molecules into preferred orientation and their stability in the aligned state up to at least cold wire bond temperatures (about 100.degree. C.).
Historically, one of the first approaches to this alignment of NLO molecules in a polymeric system came with the concept of the "guess-host" system. Singer et al., Appl. Phys. Let., 49:248 (1986). The NLO molecules may be incorporated by a solution casting method with an amorphous polymer and the second order nonlinearity may be imparted by subsequent poling of the NLO molecules in the matrix using an external electric field, e.g., corona poling, parallel plate poling or integrated electrode poling. Advantages of this approach are ease of processing, tailorable refractive indices, control of spatial ordering of the polymer, and choice of a wide range of materials. However, the decay (both the initial and long term) of second order properties as confirmed through SHG from the matrix is unavoidable when the poling field is withdrawn from the matrix. Moreover, a high degree of loading of the NLO molecules in the polymer is not possible because of phase segregation of the matrix or blooming of NLO molecules in the matrix, both resulting in optical scattering.
In a second approach, known as "grafted" systems, a number of new features are routed just by linking NLO molecules covalently in the side chains of a suitable polymer backbone. Meredith et al., Macromolecules, 15:1385 (1982). Despite the synthetic complexity of such a system, a large number of NLO molecules (a concentration 2 to 3 times greater than the guest-host system) can be coupled with the polymer side chains, yet the polymers are easily processable. Both the initial and long term decay in second harmonic (SH) properties are reduced to a great extent.
Recently, a three dimensional network consisting of NLO molecules, known as the "cross-linked" system, has been developed to overcome a number of problems associated with the guest-host or grafted systems. Reck et al., SPIE, 1147:74 (1989) and Eich et al., J. Appl. Phys., 66(7):3241 (1989). In this system, multifunctional epoxy and amino compounds containing NLO components are simultaneously processed, poled and crosslinked to freeze-in the nonlinear effects permanently. Properties resulting from the cross-linked system are significantly small decay in SH properties over a long period of time and the ability for processing with large concentrations of NLO molecules. However, for developing an optimal epoxy based NLO material precise control of the molecular weight of the prepolymer is a stringent and necessary condition. In addition, poling and curing at elevated temperatures has to be carried out over a long period of time (about 20 hours) making processing of the materials significantly difficult.